The present invention relates to a high-power ultraviolet laser light generating apparatus having a frequency converter.
There has been an ultraviolet radiation generating apparatus which employs green laser light or ultraviolet laser light as incident light and converts a frequency of the laser light by using a frequency conversion device made of a nonlinear optical crystal to thereby generate ultraviolet laser light or ultraviolet laser light having a shorter wavelength.
A .beta.-BBO (.beta.-BaB.sub.2 O.sub.4) crystal that is a nonlinear optical crystal is frequently used to manufacture the frequency conversion device used in such ultraviolet radiation generating apparatus. Such BBO crystal has no deliquescence and chemical stability and can easily be processed in the air, and hence such BBO crystal is used to manufacture the frequency conversion device used for stably generating ultraviolet radiation. However, when the frequency conversion device made of the BBO crystal is used to generate high-power ultraviolet radiation having a mean output power of 1 kW/cm.sup.2 or higher, a region, from which ultraviolet radiation is irradiated, at an end surface of the BBO crystal is changed in property as time passes. As a result, within a short period of time, i.e., several tens of hours, an optical damage which drastically reduces the ultraviolet light output is produced at the region. Moreover, a so-called Fresnel reflection loss resulting from surface reflection at both of end surfaces of the BBO crystal leads to reflection loss of about 20% of the ultraviolet light output.
As described above, when the ultraviolet radiation generating apparatus generates ultraviolet radiation with a high output power such as a mean output power of 1 kW/cm.sup.2 or higher, change in property of the frequency conversion device, i.e., the nonlinear optical crystal becomes a bar to high-output generation of ultraviolet radiation although it does not become a bar to an ordinary low-power generation thereof.
The inventors of the present invention et al. studied and researched the change in property of the nonlinear optical crystal and found that slight deliquescence in the linear optical crystal, e.g., BBO crystal leads to the change in property which becomes a bar to generation of short-wavelength, i.e., high-energy ultraviolet laser light at high output power.